The present invention relates generally to molds suitable for use in forming cube corner retroreflective sheeting and to methods for making the same. In particular, the present invention relates to molds formed from a plurality of thin laminae and to methods for making the same.
Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the wide-spread use of retroreflective sheeting in a variety of conspicuity applications. Retroreflective sheeting is frequently used on flat, rigid articles such as, for example, road signs and barricades; however, it is also used on irregular or flexible surfaces. For example, retroreflective sheeting can be adhered to the side of a truck trailer, which requires the sheeting to pass over corrugations and protruding rivets, or the sheeting can be adhered to a flexible body portion such as a road worker""s safety vest or other such safety garment. In situations where the underlying surface is irregular or flexible, the retroreflective sheeting desirably possesses the ability to conform to the underlying surface without sacrificing retroreflective performance. Additionally, retroreflective sheeting is frequently packaged and shipped in roll form, thus requiring the sheeting to be sufficiently flexible to be rolled around a core.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as xe2x80x9cbeadedxe2x80x9d sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult). Advantageously, microsphere-based sheeting can generally be adhered to corrugated or flexible surfaces. Also, due to the symmetrical geometry of beaded retroreflectors, microsphere based sheeting exhibits a relatively orientationally uniform total light return when rotated about an axis normal to the surface of the sheeting. Thus, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar base surface and a structured surface comprising a plurality of cube corner elements opposite the base surface. Each cube corner element comprises three mutually substantially perpendicular optical faces that intersect at a single reference point, or apex. The base of the cube corner element acts as an aperture through which light is transmitted into the cube corner element. In use, light incident on the base surface of the sheeting is refracted at the base surface of the sheeting, transmitted through the bases of the cube corner elements disposed on the sheeting, reflected from each of the three perpendicular cube corner optical faces, and redirected toward the light source. The symmetry axis, also called the optical axis, of a cube corner element is the axis that extends through the cube corner apex and forms an equal angle with the three optical faces of the cube corner element. Cube corner elements typically exhibit the highest optical efficiency in response to light incident on the base of the element roughly along the optical axis. The amount of light retroreflected by a cube corner retroreflector drops as the incidence angle deviates from the optical axis.
The maximum retroreflective efficiency of cube corner retroreflective sheeting is a function of the geometry of the cube corner elements on the structured surface of the sheeting. The terms xe2x80x98active areaxe2x80x99 and xe2x80x98effective aperturexe2x80x99 are used in the cube corner arts to characterize the portion of a cube corner element that retroreflects light incident on the base of the element. A detailed teaching regarding the determination of the active aperture for a cube corner element design is beyond the scope of the present disclosure. One procedure for determining the effective aperture of a cube corner geometry is presented in Eckhardt, Applied Optics, v. 10, n. 7, July, 1971, pp. 1559-1566. U.S. Pat. No. 835,648 to Straubel also discusses the concept of effective aperture. At a given incidence angle, the active area can be determined by the topological intersection of the projection of the three cube corner faces onto a plane normal to the refracted incident light with the projection of the image surfaces for the third reflections onto the same plane. The term xe2x80x98percent active areaxe2x80x99 is then defined as the active area divided by the total area of the projection of the cube corner faces. The retroreflective efficiency of retroreflective sheeting correlates directly to the percentage active area of the cube corner elements on the sheeting.
Additionally, the optical characteristics of the retroreflection pattern of retroreflective sheeting are, in part, a function of the geometry of the cube corner elements. Thus, distortions in the geometry of the cube corner elements can cause corresponding distortions in the optical characteristics of the sheeting. To inhibit undesirable physical deformation, cube corner elements of retroreflective sheeting are typically made from a material having a relatively high elastic modulus sufficient to inhibit the physical distortion of the cube corner elements during flexing or elastomeric stretching of the sheeting. As discussed above, it is frequently desirable that retroreflective sheeting be sufficiently flexible to allow the sheeting to be adhered to a substrate that is corrugated or that is itself flexible, or to allow the retroreflective sheeting to be wound into a roll to facilitate storage and shipping.
Cube corner retroreflective sheeting is manufactured by first manufacturing a master mold that includes an image, either negative or positive, of a desired cube corner element geometry. The mold can be replicated using nickel electroplating, chemical vapor deposition or physical vapor deposition to produce tooling for forming cube corner retroreflective sheeting. U.S. Pat. No. 5,156,863 to Pricone, et al. provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and laminate techniques. Each of these techniques has benefits and limitations.
In pin bundling technique, a plurality of pins, each having a geometric shape on one end, are assembled together to form a cube-corner retroreflective surface. U.S. Pat. No. 1,591,572 (Stimson), U.S. Pat. No. 3,926,402 (Heenan), 3,541,606 (Heenan et al.) and U.S. Pat. No. 3,632,695 (Howell) provide illustrative examples. Pin bundling techniques offer the ability to manufacture a wide variety of cube corner geometries in a single mold. However, pin bundling techniques are economically and technically impractical for making small cube corner elements (e.g. less than about 1.0 millimeters).
In direct machining techniques, a series of grooves is formed in a unitary substrate to form a cube-corner retroreflective surface. U.S. Pat. No. 3,712,706 (Stamni) and U.S. Pat. No. 4,588,258 (Hoopman) provide illustrative examples. Direct machining techniques offer the ability to accurately machine very small cube corner elements which are compatible with flexible retroreflective sheeting. However, it is not presently possible to produce certain cube corner geometries that have very high effective apertures at low entrance angles using direct machining techniques. By way of example, the maximum theoretical total light return of the cube corner element geometry depicted in U.S. Pat. No. 3,712,706 is approximately 67%.
In laminate techniques, a plurality of laminae, each lamina having geometric shapes on one end, are assembled to form a cube-corner retroreflective surface. German Provisional Publication (OS) 19 17 292, International Publication Nos. WO 94/18581 (Bohn, et al.), WO 97/04939 (Mimura et al.), and WO 97/04940 (Mimura et al.), disclose a molded reflector wherein a grooved surface is formed on a plurality of plates. The plates are then tilted by a certain angle and each second plate is shifted crosswise. This process results in a plurality of cube corner elements, each element formed by two machined surfaces on a first plate and one side surface on a second plate. German Patent DE 42 36 799 to Gubela discloses a method for producing a molding tool with a cubical surface for the production of cube corners. An oblique surface is ground or cut in a first direction over the entire length of one edge of a band. A plurality of notches are then formed in a second direction to form cube corner reflectors on the band. Finally, a plurality of notches are formed vertically in the sides of the band. German Provisional Patent 44 10 994 C2 to Gubela is a related patent. The reflectors disclosed in Patent 44 10 994 C2 are characterized by the reflecting surfaces having concave curvature.
The present invention relates to providing a master mold suitable for use in forming retroreflective sheeting from a plurality of laminae and methods of making the same. Advantageously, master molds manufactured according to methods disclosed herein enable the manufacture of retroreflective cube corner sheeting that exhibits retroreflective efficiency levels approaching 100%. To facilitate the manufacture of flexible retroreflective sheeting, the disclosed methods enable the manufacture of cube corner retroreflective elements having a width of 0.010 millimeters or less. Additionally, the present application enables the manufacture of a cube corner retroreflective sheeting that exhibits symmetrical retroreflective performance in at least two different orientations.
Efficient, cost-effective methods of making molds formed from a plurality of laminae are also disclosed. In particular, a reduction of the number of laminae necessary to produce a given density of cube corner elements in a sheeting is disclosed, thereby reducing the time and expense associated with manufacturing such molds.
In one embodiment, a lamina suitable for use in a mold for use in forming retroreflective cube corner articles is provided, the lamina having opposing first and second major surfaces defining therebetween a first reference plane, the lamina further including a working surface connecting the first and second major surfaces, the working surface defining a second reference plane substantially parallel to the working surface and perpendicular to the first reference plane and a third reference plane perpendicular to the first reference plane and the second reference plane. The lamina includes: (a) a first groove set including at least two parallel adjacent V-shaped grooves in the working surface of the lamina, each of the adjacent grooves defining a first groove surface and a second groove surface that intersect substantially orthogonally to form a first reference edge; (b) a second groove set including at least two parallel adjacent V-shaped grooves in the working surface of the lamina, each of the adjacent grooves defining a third groove surface and a fourth groove surface that intersect substantially orthogonally to form a second reference edge; and (c) a third groove set including at least one groove in the working surface of the lamina, the groove defining a fifth groove surface and a sixth groove surface, the fifth groove surface intersecting substantially orthogonally with the first and second groove surfaces to form at least one first cube corner disposed in a first orientation and the sixth groove surface intersecting substantially orthogonally with the third and fourth groove surfaces to form at least one second cube corner disposed in a second orientation different than the first orientation.
In one embodiment, the first and second groove sets are formed such that their respective reference edges extend along axes that, in a top plan view, are perpendicular to the first reference plane. The third groove set includes a single groove having a vertex that extends along an axis contained by the third reference plane. In this embodiment, the lamina comprises a first row of cube corner elements defined by the grooves of the first groove set and the third groove and a second row of cube corner elements defined by the grooves of the second groove set and the third groove.
The three mutually perpendicular optical faces of each cube corner element are preferably formed on a single lamina. All three optical faces are preferably formed by the machining process to ensure optical quality surfaces. A planar interface is preferably maintained between adjacent first and second major surfaces during the machining phase and subsequent thereto so as to minimize alignment problems and damage due to handling of the laminae.
Also disclosed is a method of manufacturing a lamina for use in a mold suitable for use in forming retroreflective cube corner articles, the lamina having opposing first and second major surfaces defining therebetween a first reference plane, the lamina further including a working surface connecting the first and second major surfaces, the working surface defining a second reference plane substantially parallel to the working surface and perpendicular to the first reference plane and a third reference plane perpendicular to the first reference plane and the second reference plane. The method includes: (a) forming a first groove set including at least two parallel adjacent V-shaped grooves in the working surface of the lamina, each of the adjacent grooves defining a first groove surface and a second groove surface that intersect substantially orthogonally to form a first reference edge; (b) forming a second groove set including at least two parallel adjacent V-shaped grooves in the working surface of the lamina, each of the adjacent grooves defining a third groove surface and a fourth groove surface that intersect substantially orthogonally to form a second reference edge; and (c) forming a third groove set including at least one groove in the working surface of the lamina, the groove defining a fifth groove surface and a sixth groove surface, the fifth groove surface intersecting substantially orthogonally with the first and second groove surfaces to form at least one first cube corner disposed in a first orientation and the sixth groove surface intersecting substantially orthogonally with the third and fourth groove surfaces to form at least one second cube corner disposed in a second orientation different than the first orientation.
Further disclosed is a mold assembly comprising a plurality of laminae, the laminae including opposed parallel first and second major surfaces defining therebetween a first reference plane, each lamina further including a working surface connecting the first and second major surfaces, the working surface defining a second reference plane substantially parallel to the working surface and perpendicular to the first reference plane and a third reference plane perpendicular to the first reference plane and the second reference plane. The working surface of a plurality of the learning includes: (a) a first groove set including at least two parallel adjacent V-shaped grooves in the working surface of each of the laminae, each of the adjacent grooves defining a first groove surface and a second groove surface that intersect substantially orthogonally to form a first reference edge on each of the respective laminae; (b) a second groove set including at least two parallel adjacent V-shaped grooves in the working surface of each of the learning, each of the adjacent grooves defining a third groove surface and a fourth groove surface that intersect substantially orthogonally to form a second reference edge on each of the respective laminae; and (c) a third groove set including at least one groove in the working surface of a plurality of the laminae, each groove defining a fifth groove surface and a sixth groove surface, the fifth groove surface intersecting substantially orthogonally with the first and second groove surfaces to form at least one first cube corner disposed in a first orientation and the sixth groove surface intersecting substantially orthogonally with the third and fourth groove surfaces to form at least one second cube corner disposed in a second orientation different than the first orientation.
In one embodiment of such mold assembly, the first groove set extends substantially entirely across the respective first major surfaces of the plurality of laminae and the second groove get extends substantially entirely across the respective second major surfaces of the plurality of laminae. Additionally, the first and second groove sets are formed such that their respective reference edges extend along axes that, in a top plan view, are perpendicular to the respective first reference planes. Finally, the third groove set comprises a single groove in each respective lamina having a vertex that extends along an axis parallel to the respective lamina""s third reference plane. According to this embodiment, each respective lamina comprises a first row of cube corner elements defined by the grooves of the first groove set and the third groove and a second row of cube corner elements defined by the grooves of the second groove set and the third groove.
Also disclosed is a method of manufacturing a plurality of laminae for use in a mold suitable for use in forming retroreflective cube corner articles, each lamina having opposing first and second major surfaces defining therebetween a first reference plane, each lamina further including a working surface connecting the first and second major surfaces, the working surface defining a second reference plane substantially parallel to the working surface and perpendicular to the first reference plane and a third reference plane perpendicular to the first reference plane and the second reference plane. The method includes: (a) orienting a plurality of laminae to have their respective first reference planes parallel to each other and disposed at a first angle relative to a fixed reference axis; (b) forming a first groove set including at least two parallel adjacent V-shaped grooves in the working surface of each of the laminae, each of the adjacent grooves defining a first groove surface and a second groove surface that intersect substantially orthogonally to form a first reference edge on each of the respective laminae; (c) orienting the plurality of laminae to have their respective first reference planes parallel to each other and disposed at a second angle relative to the fixed reference axis; (d) forming a second groove set including at least two parallel adjacent V-shaped grooves in the working surface of each of the laminae, each of the adjacent grooves defining a third groove surface and a fourth groove surface that intersect substantially orthogonally to form a second reference edge on each of the respective laminae; and (e) forming a third groove set including at least one groove in the working surface of a plurality of the laminae, each groove defining a fifth groove surface and a sixth groove surface, the fifth groove surface intersecting substantially orthogonally with the first and second groove surfaces to form at least one first cube corner disposed in a first orientation and the sixth groove surface intersecting substantially orthogonally with the third and fourth groove surfaces to form at least one second cube corner disposed in a second orientation different than the first orientation.
In one disclosed method, the plurality of laminae are assembled in a suitable fixture that defines a base plane. The fixture secures the laminae such that their respective first reference planes are substantially parallel and are disposed at a first angle that preferably measures between about 1xc2x0 and about 85xc2x0, and more preferably measures between about 10xc2x0 and about 60xc2x0 relative to a fixed reference axis that is a normal vector to the base plane. The first groove set is then formed by removing portions of each of the plurality of lamina proximate the working surface of the plurality of laminae by using a suitable material removal technique such as, for example, ruling, fly-cutting, grinding, or milling. The plurality of laminae are then reassembled in the fixture and secured such that their respective first reference planes are substantially parallel and are disposed at a second angle that preferably measures between about 1xc2x0 and about 85xc2x0, and more preferably measures between about 10xc2x0 and about 60xc2x0 relative to a fixed reference axis that is a normal vector to the base plane. The second groove set is then formed using suitable material removal techniques as describe above. The plurality of laminae are then reassembled in the fixture and secured such that their respective first reference planes are substantially parallel to the reference axis. The third groove set is then formed using suitable material removal techniques as describe above. Preferably, the third groove set defines a single groove in each respective lamina.